transport: add got_remote_refs flag

transport_get_remote_refs() in tranport.c checks transport->remote_refs
to determine whether transport->get_refs_list() should be invoked.  The
logic is "if it is NULL, we haven't run ls-remote to find out yet".

However, transport->remote_refs could still be NULL while cloning from
an empty repository.  This causes get_refs_list() to be run unnecessarily.

Introduce a flag, transport->got_remote_refs, to more explicitly record
if we have run transport->get_refs_list() already.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

transport.c

@@ -918,6 +918,7 @@ struct transport *transport_get(struct remote *remote, const char *url)
 	if (!remote)
 		die("No remote provided to transport_get()");
 
+	ret->got_remote_refs = 0;
 	ret->remote = remote;
 	helper = remote->foreign_vcs;
 
@@ -1079,8 +1080,10 @@ int transport_push(struct transport *transport,
 
 const struct ref *transport_get_remote_refs(struct transport *transport)
 {
-	if (!transport->remote_refs)
+	if (!transport->got_remote_refs) {
 		transport->remote_refs = transport->get_refs_list(transport, 0);
+		transport->got_remote_refs = 1;
+	}
 
 	return transport->remote_refs;
 }

transport.h

@@ -19,6 +19,12 @@ struct transport {
 	void *data;
 	const struct ref *remote_refs;
 
+	/**
+	 * Indicates whether we already called get_refs_list(); set by
+	 * transport.c::transport_get_remote_refs().
+	 */
+	unsigned got_remote_refs : 1;
+
 	/**
 	 * Returns 0 if successful, positive if the option is not
 	 * recognized or is inapplicable, and negative if the option